You may have an illness and not even know it—and the proof could be on your breath. That’s because our bodies give off complex aromas, some too faint for a human nose. And when sickness alters cell metabolism, an array of telltale volatile chemicals make their way into our breath, urine, and sweat. Now, research labs around the world are figuring out how to use gas sensors to detect disease. Here are a few ailments they’ve gotten a whiff of so far.

Breath odor: Rotten apples, nail polish remover
Source: Acetone
Detection: Polymer sensors. Acetone is produced by the liver when sugar-starved cells shred fat to make energy. In theory, the sensors could be built into cheap, pocket-sized gadgets that tell diabetics when to take insulin.

Schizophrenia

Breath odor: Ether
Source: Carbon disulphide
Detection: Ion trap mass spectrometer. If someone is certifiably nuts, their breath may have notes of ether and pentane, according to one study. Unfortunately pentane appears in high levels with all sorts of disorders, so it’s not a reliable marker.